Part 3

The winds near the ground are modified both in velocity and direction by friction. The free flow is often interfered with by topography.

THE TRUE AIR FLOW

One must rise above the ground some distance to get the true air flow, or what is known as the gradient wind, the flow which balances the gradient, i.e. a flow along the isobars. The gradient velocity is found about 300 metres above the ground, and the gradient direction a little higher. The lower clouds as a rule indicate true wind values very well; and so, it is desirable in studying winds to use cloud directions and velocities rather than surface values. In cloud work a nephoscope is essential. The unaided eye, unless properly shielded, suffers from the glare of a sunlit sky; and moreover, there are no fixed points or references. A black mirror, with suitable sighting rods and measuring devices, enables an observer to follow the cloud, estimate its height and determine with accuracy the direction from which it is moving. There is an average difference of 30 degrees between the cloud direction and the surface wind; the upper direction being more to the right. At times the directions may be opposite.

It may seem surprising but few of us, except at sunrise and sunset, really see what is going on in cloud land.

Some meteorologists hold that the circulation of air 3000 to 5000 metres above the ground controls the path and perhaps the intensity of storms. It is therefore important to know something of the flow at high levels if we would improve the forecasts.

LIMITATIONS OF MAP

The weather map fails to indicate what shifts of direction and changes in velocity are likely to occur. The forecaster tries to anticipate these, but he bases his conclusions chiefly upon an expected movement of the low area; using the accumulated records of the paths of past storms. But each storm is in reality a law unto itself; and while we know something of the relations between pressure and flow of the air; as yet we know very little about the relations of wind and weather. The problem is complicated by the behavior of the load of water vapor.

The Chief Forecaster of one of the great national weather services recently wrote:

"Despite the fact that maps have now been drawn day by day for over half a century, we may safely say that no two maps have been identical."

It is perhaps unfortunate that so much attention has been given to the cyclone or depression or LOW, and comparatively little to the HIGH or anticyclone. For we are now beginning to understand that while there may seem at first to be nothing specially noteworthy about a mass of air where the pressure varies from 1020 to 1040 kilobars, that is, 2 to 4 per cent above a standard atmosphere, with isobars irregularly curved and feeble surface winds, yet the anticyclone is more important than the cyclone in determining weather sequence; for the progressive motion of the cyclone depends largely upon the strength of the anticyclone.

OCEAN STORMS

Sir Napier Shaw, who has written much on the weather of the British Isles, may be quoted here.

"Anyone who is interested in the weather is always on the lookout for 'lows' and is very keen to know whether he is going to be on the south of the centre or the north of it. He is, of course, interested in the anticyclone too, because as long as an anticyclone is there, there cannot be a depression; but it is the depression which has the life and movement about it, giving it a claim to the attention of everybody who wants to know what the weather and its changes are going to be.

"This has been recognized from the very earliest days of weather maps with isobars. The depressions which pass over our shores (Great Britain) mostly come from the west. Some of them come all the way from America; one or two have been traced from the west coast of Africa and so have crossed the Atlantic twice, first to the westward and then to the eastward. Some have come all the way from a sort of parent 'low' in the North Pacific Ocean. So general is the tendency for 'lows' to go eastward that it was thought at one time, particularly by the 'New York Herald,' that their departure from the American Coast and subsequent arrival on our own shores could be notified by cable, and we (the British) might thus be forewarned of their approach, some three or four days in advance. The attempt was made by the 'New York Herald' acting in co-operation with the Meteorological Offices of the United Kingdom and France. But a depression keeps to no beaten track; it has as many paths for its centre as there are lines in a bundle of hay. Though groups can be picked out there are many strays, and, moreover, the depression changes its shape and intensity while it travels, so that if you lose sight of it for a day you cannot be at all sure of its identity."

TRANSCONTINENTAL STORMS

If there is so much uncertainty in forecasting the path of a disturbance at sea, how much more uncertain must it be on land? Elaborate statistics of the average daily movement of various types of storms have been officially published. The average speed of storms (not wind speeds) across the United States is 11 metres per second or 25 miles an hour. Storms travel more rapidly in winter than in summer, about half again as fast; that is, summer storms travel 20 miles, and winter storms 30 miles, an hour.

The paths vary widely; from the Gulf storms moving northeast and West Indian hurricanes recurving on the southern coast, to the storms from Alberta and the west which move south and east. Ten types of storms, classified according to the place of origin, are recognized by the official forecasters of the United States. These are North Pacific, Alberta, Northern Rocky Mountain, Colorado, Central, South Pacific, Texas, East Gulf, South Atlantic and West Indian Hurricanes. A better nomenclature would be (1) Alberta, (2) Washington, (3) Kootenay, (4) Utah, (5) Kansas, (6) California, (7) Texas, (8) Louisiana, (9) Florida, and (10) Hurricanes.

HURRICANES

Type 10 is the general class of tropical storms occurring chiefly in the summer and fall which, drifting west, slowly work northward. Similar storms are the typhoons and baguios of the East Indian and China Seas.

The path and point of recurvature will be determined by the position of the Bermuda Hyperbar, that is, the seasonal anticyclone of the Atlantic. This accounts for the swinging east and north of these tracks as the season progresses; for the hyperbar is slowly displaced east, the maximum displacement occurring in September.

FIG. 8. ALL STORMS LEAD TO NEW ENGLAND]

Individual anticyclones also influence individual hurricanes. Thus a hurricane passing west over Havana, will go farther west if a vigorous "high" is spreading southeast over the Gulf States. And when this "high" passes seaward, the hurricane will work around the southwest quadrant of the "high," recurving and moving northeast.

STORM RENDEZVOUS

Altho storms originate or are first detected in nine different sections, it is a fact worth mentioning that they all leave the United States in the vicinity of New England or Nova Scotia. Some of the southern depressions starting near the coast, pass to sea south of New York, but in general an observer standing on Plymouth Rock can virtually encompass within a radius of 500 kilometres, 300 miles, the paths of ninety per cent of the storms that traverse the country.

Thus a storm that originates in Texas (7) will probably pass close to Cape Cod. Likewise, types (3) and (5); while the other types may pass a little to the north or south. See Chart, Paths of Storms.

STORM PATHS

Forecasting then would seem to be very easy; for one would only have to know the place of origin of the storm and the rate of travel, to foretell exactly the time of arrival. Unfortunately these are only the average paths; and as with most mean values, represent a value not often experienced in fact. These paths then are not paths which any given storm will follow. One must recall the story of the operating surgeon who gave the average age of his patients in the operating room as 35. There were but two patients, one 69 years old and the other 1 year old.

As a matter of fact the path of any individual depression depends upon several factors, some of which are:--the prevailing eastward drift of the air; the extent and motion of some anticyclone advancing before the "LOW"; the duration and speed of relatively dry cold tongues of air from the north; and the supply of water vapor brought from southern waters by south winds. A depression can make little headway if to the north or east the normal path is blocked by what is known as a stagnant "HIGH." So therefore, if the anticyclone is a slow mover, a Texas storm, which would normally pass not far from southern New England, may be deflected farther north than when the HIGH moved rapidly east. So too, with the storms which originate in the western part of the country. A slow moving HIGH will prevent the LOW following it, from moving east at a normal rate along the usual path.